NAME

btrfsslower - Trace slow btrfs file operations, with per-event details.

SYNOPSIS

btrfsslower [-h] [-j] [-p PID] [min_ms] [-d DURATION]

DESCRIPTION

This tool traces common btrfs file operations: reads, writes, opens, and syncs. It measures the time spent in these operations, and prints details for each that exceeded a threshold.

WARNING: See the OVERHEAD section.

By default, a minimum millisecond threshold of 10 is used. If a threshold of 0 is used, all events are printed (warning: verbose).

Since this works by tracing the btrfs_file_operations interface functions, it will need updating to match any changes to these functions.

Since this uses BPF, only the root user can use this tool.

REQUIREMENTS

CONFIG_BPF and bcc.

OPTIONS

-p PID Trace this PID only.

min_ms: Minimum I/O latency (duration) to trace, in milliseconds. Default is 10 ms.
-d DURATION: Total duration of trace in seconds.

EXAMPLES

Trace synchronous file reads and writes slower than 10 ms:: # btrfsslower
Trace slower than 1 ms:: # btrfsslower 1
Trace slower than 1 ms, and output just the fields in parsable format (csv):: # btrfsslower -j 1
Trace all file reads and writes (warning: the output will be verbose):: # btrfsslower 0
Trace slower than 1 ms, for PID 181 only:: # btrfsslower -p 181 1
Trace for 10 seconds only:: # btrfsslower -d 10

FIELDS

TIME(s): Time of I/O completion since the first I/O seen, in seconds.
COMM: Process name.
PID: Process ID.
T: Type of operation. R == read, W == write, O == open, S == fsync.
OFF_KB: File offset for the I/O, in Kbytes.
BYTES: Size of I/O, in bytes.
LAT(ms): Latency (duration) of I/O, measured from when it was issued by VFS to the filesystem, to when it completed. This time is inclusive of block device I/O, file system CPU cycles, file system locks, run queue latency, etc. It's a more accurate measure of the latency suffered by applications performing file system I/O, than to measure this down at the block device interface.
FILENAME: A cached kernel file name (comes from dentry->d_name.name).
ENDTIME_us: Completion timestamp, microseconds (-j only).
OFFSET_b: File offset, bytes (-j only).
LATENCY_us: Latency (duration) of the I/O, in microseconds (-j only).

OVERHEAD

This adds low-overhead instrumentation to btrfs writes and fsyncs, as well as all system reads and opens (due to the current implementation of the btrfs_file_operations interface). Particularly, all reads and writes from the file system cache will incur extra overhead while tracing. Such reads and writes can be very frequent (depending on the workload; eg, 1M/sec), at which point the overhead of this tool may become noticeable. Measure and quantify before use. If this continues to be a problem, consider switching to a tool that prints in-kernel summaries only, such as btrfsdist(8).

Note that the overhead of this tool should be less than fileslower(8), as this tool targets btrfs functions only, and not all file read/write paths (which can include socket I/O).

SOURCE

This is from bcc.

: https://github.com/iovisor/bcc

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.

OS

Linux

STABILITY

Unstable - in development.

AUTHOR

Brendan Gregg